1. Field of the Present Invention
This invention relates to the field of integrated circuits and in particular to a temperature compensated exponential gain control voltage for an automatic gain control (AGC) amplifier.
2. Background Art
Automatic gain control circuits are used to maintain a constant amplitude signal output level regardless of the level of an input signal. In other words, although the amplitude of an input signal level may vary, the output of the AGC circuit is a constant amplitude. Whereas a typical electronic amplifier accepts a incoming electronic signal and amplifies that signal by a fixed amount, a AGC provides only enough amplification, or "gain", to reach the desired amplitude level.
One use for AGC circuits is in the read channels, such as found in a disk drive or similar application. Signals picked up off the magnetic media varying in amplitude are amplified by a head amplifier and then provided to the AGC circuit. It is desired that the input signal be amplified to a fixed level for subsequent processing and decoding. Therefore, the AGC amplifies the input signals to a fixed output level.
In operation the gain of an AGC circuit amplifier stage varies inversely to the amplitude of the input signal level. As the amplitude of the input signal increases, the amount of gain is reduced. The AGC is initialized at a predetermined reference level which the AGC seeks to maintain. When the amplitude signal falls below this predetermined threshold level, the AGC circuit senses the decrease in amplitude at the output of the amplifier. The AGC circuit then increases the amplifier stage gain until the amplitude of the output signal increases to the reference level. If the amplitude of the input signal is above the nominal reference level, the AGC circuit senses the increase in amplitude at the output of the amplifier stage and decreases the amplifier stage gain until the amplitude of the output signal decreases to the reference level.
A typical AGC circuit consists of an input stage for receiving a input signal. The input signal is provided to an amplifying stage where it is multiplied by a gain factor to increase the amplitude of the input signal. This amplifying stage produces an output signal of the circuit. A detection stage detects the amplitude of the output signal and compares it to a reference level or a reference range. If the amplitude of the input signal is not at the desired level, a error signal is provided to a correction stage which adjusts the gain of the amplifying stage so that the amplitude of the output signal is at or within the desired range. In some prior art implementations, a switched capacitor array is utilized to provide variable gain such as is described in U.S. Pat. No. 4,691,172 assigned to the assignee to the present invention.
The gain versus input control voltage for a typical AGC amplifier is usually an exponential function which is required in the feedback loop characteristics to achieve a rapid gain settling time. In the prior art, the exponential function is implemented using a emitter coupled pair of transistors in a configuration such as a Gilbert Multiplier. One disadvantage of such an implementation is that the exponential function is highly temperature variant. If a user wishes to drive this voltage with a digital-to-analog converter (DAC), then the DAC would also have to have the same temperature variation to compensate. If one voltage is not compensated, the temperature dependency results in the requirement that the user must supply different voltages to get a specific gain over a temperature range. This adds complexity to the circuitry and software.
Therefore, it is an object of the present invention to provide an automatic gain control circuit in which gain versus input control voltage is temperature independent.
It is yet another object of the present invention to provide an automatic gain control circuit with reduced circuitry and software requirements.